Hemoproteins constitute a diverse class of proteins. The functions of many of these proteins involves the binding of a ligand to the heme prosthetic group. Numerous studies have described the binding of anionic ligands to iron in ferric hemoproteins in an attempt to provide information relevant to hemoprotein function in biological systems. These studies have suggested that several structural parameters contribute to the ligand binding properties of a hemoprotein. The extent to which each of these parameters effect the ligand binding properties has, however, not been determined. The proposed research seeks to evaluate the importance of these structural factors by characterizing the ligand binding properties of appropriate model systems. Recent studies in this laboratory have provided the first data on anion binding to a model ferric heme-histidine complex for comparison to hemoproteins. This project will measure the binding constants of other ligands as well as the ligand binding properties of other model heme complexes. Equilibrium constants and derived thermodynamic values will be compared to reported values for hemoproteins. The results will provide a basis for evaluating the magnitude as well as the manner by which protein structure influences the ligand binding properties of hemoproteins. The results are relevant to the understanding of the functional diversity of hemoproteins and the structural basis for the properties of abnormal hemoproteins.